From language/operators

See Original text in context

sub infix:<^fff^>(Mu $a, Mu $b)

Like fff, except it does not return true for matches to either the left or right argument.

my @list = <A B C>;
say $_ if /A/ fff /C/ for @list;   # OUTPUT: «A␤B␤C␤» 
say $_ if /A/ ^fff^ /C/ for @list; # OUTPUT: «B␤»

For the non-sed version, see ^ff^.

This operator cannot be overloaded, as it's handled specially by the compiler.